In a cellular communication system a geographical region is divided into a number of cells each of which is served by a base station. The base stations are interconnected by a fixed network which can communicate data between the base stations. A mobile station is served via a radio communication link by the base station of the cell within which the mobile station is situated.
As a mobile station moves, it may move from the coverage of one base station to the coverage of another, i.e. from one cell to another. As the mobile station moves towards a base station, it enters a region of overlapping coverage of two base stations and within this overlap region it changes to be supported by the new base station. As the mobile station moves further into the new cell, it continues to be supported by the new base station. This is known as a handover or handoff of a mobile station between cells.
A typical cellular communication system extends coverage over typically an entire country and comprises hundreds or even thousands of cells supporting thousands or even millions of mobile stations. Communication from a mobile station to a base station is known as uplink, and communication from a base station to a mobile station is known as downlink.
The fixed network interconnecting the base stations is operable to route data between any two base stations, thereby enabling a mobile station in a cell to communicate with a mobile station in any other cell. In addition, the fixed network comprises gateway functions for interconnecting to external networks such as the Public Switched Telephone Network (PSTN), thereby allowing mobile stations to communicate with landline telephones and other communication terminals connected by a landline. Furthermore, the fixed network comprises much of the functionality required for managing a conventional cellular communication network including functionality for routing data, admission control, resource allocation, subscriber billing, mobile station authentication etc.
Currently, the most ubiquitous cellular communication system is the 2nd generation communication system known as the Global System for Mobile communication (GSM). Further description of the GSM TDMA communication system can be found in ‘The GSM System for Mobile Communications’ by Michel Mouly and Marie Bernadette Pautet, Bay Foreign Language Books, 1992, ISBN 2950719007.
3rd generation systems are currently being rolled out to further enhance the communication services provided to mobile users. One such system is the Universal Mobile Telecommunication System (UMTS), which is currently being deployed. Further description of CDMA and specifically of the Wideband CDMA (WCDMA) mode of UMTS can be found in ‘WCDMA for UMTS’, Harri Holma (editor), Antti Toskala (Editor), Wiley & Sons, 2001, ISBN 0471486876. Third generation cellular communication systems are standardized in the 3rd Generation Partnership Project (3GPP).
In a cellular communication system, user equipments attach to one (or more) base stations wirelessly. User equipments attach to base stations according to parameters such as signal quality for the base station, system information signaled from the base station (where the system information can contain parameters such as the identity of the network operator), handover commands from the network (where a base station may force a user equipment to attach to a different network due to issues such as relative signal quality, traffic load on base stations etc.) etc.
FIG. 1 illustrates an example of user equipments (UEs) attaching to first and second base stations of two different networks. The figure further illustrates that the two networks broadcast system information to all user equipments in the geographical area covered by the networks. The user equipments attach to base stations, not only on the basis of signal strength etc., but also on the basis of a network identifier that is broadcast in this system information (hence subscribers to the first network only attach to base stations of the first network and subscribers of the second network only attach to base stations of the second network). The dashed lines in FIG. 1 illustrate the attachments between the user equipments and the base stations.
In 3GPP, a user equipment searching for a cell to attach to will generally attempt to attach to the cell from a preconfigured list that meets certain quality criteria (such as signal strength). The user equipment may for example comprise a preconfigured list of possible frequencies for suitable candidate cells (these frequencies can be programmed in a Subscriber Identity Module (SIM) where the SIM allows an operator to customize the user equipment to only search the frequencies that belong to that network operator). When a user equipment has identified a suitable cell, it will camp onto this and will extract the downlink frequency from the preconfigured list.
The 3GPP specifications specify that for the Frequency Duplex Division (FDD) mode of 3GPP, the uplink and downlink frequencies are paired according to an explicit relationship. Methods of signaling the uplink frequency on downlink messages are also considered for extensions of 3GPP to new frequency bands, e.g. in technical recommendation TR25.889. Thus, if the downlink frequency is known, so is the uplink frequency.
In the Time Division Duplex (TDD) mode of 3GPP, only a single frequency is used for uplink and downlink and the separation between these is achieved in the time domain. Thus, TDD utilizes an unpaired spectrum approach where the same frequency is used in both directions. In such systems, the base stations broadcast system information which contains characteristics of the random access channel and in particular this information comprises the timeslot number where RACH (Random Access CHannel) transmissions are to be sent, a list of channelization codes that are to be used for the RACH etc.
For FDD mode, the transmitted system information comprises information such as the details of the RACH preambles and available signature sequences. When the user equipment wishes to send a random access channel message, it transmits a signature sequence of length 12 timeslots. Upon receiving an acknowledgement for the preamble, the user equipment then transmits a RACH message that is either of 15 timeslots or 30 timeslots duration (i.e. it sends a RACH for either one or two whole frames).
In cellular communication systems, different duplexing modes may be used. In particular, the following two broad classifications may be used:                Full Duplex (FD) mode. In full duplex mode, the base station and the user equipment can transmit at the same time. Thus, uplink and downlink transmissions may occur simultaneously for a single user equipment. Orthogonality between the uplink and downlink is achieved by assigning the uplink for transmission on one frequency and the downlink for transmission on another frequency. Full duplex mode hence uses a paired spectrum approach.        Half Duplex (HD) mode. In half duplex mode, transmissions from a base station to a user equipment never occur simultaneously with transmissions from a user equipment to the base station. Thus, for a single user equipment, uplink and downlink transmissions are never simultaneous. Orthogonality between uplink and downlink transmissions can specifically be maintained by assigning uplink transmissions to some timeslots and downlink transmissions to other timeslots. The half duplex mode can be used with both paired and unpaired spectrum allocations:        Unpaired Spectrum. Unpaired spectrum uses a single carrier both for uplink transmissions and downlink transmissions and these are separated in time. Both the user equipment and the base station operate strictly in half duplex mode, i.e. both the user equipment and base station either transmit or receive on the carrier frequency but do not simultaneously transmit and receive.        Paired Spectrum. Paired spectrum uses a different frequency carrier for uplink and downlink. Thus, the uplink and downlink are separated in the frequency domain and is further for the individual user equipment separated in the time domain by the half duplex mode of operation. The user equipment operates in a strict half duplex mode where it either transmits or receives. The base station operates in a half duplex mode with respect to any one user equipment, i.e. it either transmits or receives to each user equipment, but can transmit and receive at the same time. Specifically, the base station may transmit to one user equipment while receiving from another user equipment but will never transmit and receive simultaneously for the same user equipment.        
The use of different duplexing schemes including paired/unpaired spectrum and half duplex/full duplex modes provides a high degree of flexibility and allows systems to be designed to meet various preferences and requirements. However, conventional systems also have a number of disadvantages.
For example, it is necessary for base stations and user equipments to be compatible with each other and thus to use the same duplexing functionality. For example, a base station may transmit system information in a way that is compatible with the duplexing scheme employed by the base station, and this may be received and decoded by a user equipment using the same duplexing scheme. However, if the user equipment uses a different duplexing scheme, it will not be able to receive the transmissions and therefore will not be able to attach to the base station.
User equipments are known which can support a number of different duplexing capabilities. Such a user equipment must monitor all the duplexing schemes that it can support in order to determine a suitable duplexing scheme for the base station to which it is attempting to attach. Such an approach is inflexible and results in high equipment complexity and complex operation.
Furthermore, known approaches do not allow a full utilization of the duplexing capabilities of the base stations and the user equipments. For example, scheduling by the network may not be able to flexibly adapt to, and select between, a plurality of different duplexing capabilities in order to optimize performance.
Hence, an improved system would be advantageous and in particular a system allowing increased flexibility, improved utilization of duplexing capabilities, improved adaptation, reduced complexity, facilitated operation, improved compatibility and/or improved performance would be advantageous.